The present invention relates to a process for the epoxidation of olefins, whereby measures are taken to ensure safe operation of the process in all stages without compromising the overall economics of the process.
It is known from EP-A 100 118 that propene can be reacted with hydrogen peroxide to yield propene oxide if titanium silicalite is used as the catalyst. A secondary reaction which always occurs to a slight extent on the titanium silicalite catalyst is the decomposition of hydrogen peroxide to form molecular oxygen. It is known from EP-A 659 473 to introduce a small amount of propene vapor into the reactor for purposes of purging any oxygen formed by hydrogen peroxide decomposition. A purge stream comprising predominantly propene and some oxygen and propene oxide is removed from the reactor.
EP-A 936 219 and EP-A 1 074 547 describe a process for regeneration of a titanium silicalite catalyst that has been deactivated in a process of epoxidation of propylene with hydrogen peroxide. In that process the reactor is discontinuously purged with an inert gas to remove propane since the regeneration of the catalyst has to be performed in the absence of the olefin. It is discussed in these references that the purging has the additional advantage that oxygen formed by the decomposition of hydrogen peroxide is also periodically removed from the reactor, thereby improving the safety of the process. But to ensure safe operation inert gas purging has to be conducted in short intervals which leads to undesired long shut-down periods.
EP-A 1 085 017 and WO 99/48883 disclose, with regard to some preferred embodiments of a process for epoxidation of propylene with hydrogen peroxide, the possibility of introducing a gas stream comprising propene and inert gas into the reactor. But there is no discussion with respect to oxygen decomposition or safety requirements.
All the prior described processes as discussed above do, however, suffer the disadvantage in common that considerable quantities of propene and propene oxide are lost together with the oxygen in the purge stream.
EP-A 719 768 and EP-A 1 122 246 describe a process for epoxidation of olefins preferably propene with hydrogen peroxide whereby a vapor stream comprising propene, propene oxide and oxygen leaving the reactor is treated in an absorption unit to recover valuable products from the vapor stream. Due to the increase of the oxygen concentration in the gas room of the absorption unit because the gas is depleted of propene and propene oxide it may be advantageous for safety reasons to introduce an inert gas stream into the absorption unit. Although these prior art documents deal with safety aspects in the absorption unit they do not address safety requirements of the entire process. Especially in the epoxidation reactor itself no measures are taken to ensure safe conduct of the process. Consequently the process can only be conducted within a narrow range of process parameters to ensure safety of the reaction with the result that there is a high risk associated with any kind of malfunction in the reaction stage.
An object of the present invention is accordingly to provide a process for the epoxidation of olefins having an improved balance of operation safety and economics of the process.
The above and other objects of the invention can be achieved by a process for the catalytic epoxidation of olefins in at least one reaction stage comprising:
(a) reacting the olefin with hydrogen peroxide in an organic, water-miscible solvent in the presence of a titanium silicalite catalyst in a reactor while continuously feeding an inert gas stream into the reactor;
(b) continuously removing an exit gas stream containing olefin oxide, unreacted olefin, oxygen and inert gas from the reactor;
(c) bringing the exit gas stream into contact in an absorption unit with the same solvent as used in the reaction stage;
(d) removing a solvent stream loaded with olefin and olefin oxide from the absorption unit; and
(e) discharging an gas stream containing oxygen and the inert gas from the absorption unit.
The present invention will be further understood with reference to the accompanying drawing which substantially represents a flow diagram of the process of this invention.